Modern telecommunications systems utilize lasers to transmit data via silica optical fibers. Two wavelength ranges are used at present--around 1550 nm and around 1330 nm. Superposing, transmitting, and then separating signals transmitted at these ranges from one another by coarse multiplexers is well known in the art.
As technology progresses, there is more and more data to be transmitted over the same optical fibers, such as cable TV, telephone, videos, and on-line services. In order to increase the quantity of data which can be transmitted, broadband systems have been developed having a plurality of channels which permit the transmission of data over lasers having wavelengths very close to one another. For this purpose, dense wavelength division multiplexers (WDM) are used. The current standard permits the use of wavelength separation of 1.6 nanometers and 0.8 nanometers from one another, which means that, in a range of 40 nm, 16 or 32 channels can be used, a channel being determined by a specified optical frequency band.
In order to maintain each laser at the proper wavelength, thereby providing acceptable system performance by eliminating long-term frequency drifts causing crosstalk between channels and facilitating channel recognition, wavelength stabilization and tuning for each of the lasers is required. The semiconductor lasers currently used fall into two categories--fixed wavelength lasers, such as DFB (distribution feedback) lasers, which are wavelength selected for a particular channel and then tuned, as with temperature, to operate at a standardized wavelength, and tunable lasers, such as DBR (distributed Bragg reflection) lasers, whose frequency can be switched or tuned to any desired frequency and stabilized in the desired channel.
A number of proposals have been made in the literature as to ways to monitor and control the wavelength of lasers in wavelength-division multiplexer systems. The most common solution is to lock each transmitter frequency to a stable optical reference, such as an Etalon filter. A synchronized Etalon filter which provides a set of equally spaced references at the standardized wavelengths, is set forth by J. H. Jang, et al., in "A Cold-Start WDM System Using a Synchronized Etalon Filter", IEEE Photonics Technology Letters, Vol. 9, No. 3, March 1997, pp 383. Other absolute references are discussed by Martin Guy, "Simultaneous Absolute Frequency Control of Laser Transmitters in both 1.3 and 1.455 .mu.m Bands for Multiwavelength Communication Systems", Journal of Lightwave Technology, Vol. 14, No. 6, June 1996, pp 1136, and by U. Kruger, et al., "Decentralized Frequency Stabilization Scheme for a Dense OFDM System Involving Simple Filters and an Absolute Reference", Journal of Lightwave Technology, Vol. 14, No. 5, May 1996, pp 649.
Other suggestions involve the use of an arrrayed-waveguide grating, such as M. Teshima, et al, "Multiwavelength simultaneous monitoring circuit employing wavelength crossover properties of arrayed-waveguide grating", Electronics Letters, Vol. 31, No. 18, Aug. 31, 1995, pp 1595, and K. Okamoto, "Fabrication of multiwavelength simultaneous monitoring device using arrayed-waveguide grating", Electronics Letters, Vol. 32, No. 6, Mar. 14, 1996, pp 569, or cascaded fibre Bragg gratings, as in C. S. Park, et al., "Frequency locking using cascaded fibre Bragg gratings in OFDM systems", Electronics Letters, Vol. 32, No. 12, Jun. 6, 1996.
A further proposal includes a frequency control scheme for multiple DBR lasers in a VWP cross-connect system described by M. Teshima, et al., "100-GHz-Spaced 8-Channel Frequency Control of DBR Lasers for Virtual Wavelength Path Cross-Connect System", IEEE Photonics Technology Letters, Vol. 8, No. 12, December 1996, pp 1701.
Alternatively, the deviations in transmission frequency can be measured at the remote node. One example is to lock the wavelength comb of a tunable DBR to a waveguide grating router to enable a laser to track and correct uncontrolled changes in a remotely located WDM device, shown by Derek Mayweather, et al., "Wavelength Tracking of Remote WDM Router in a Passive Optical Network", IEEE Photonics Technology Letters, Vol. 8, No. 9, September 1996, pp 1238. Another is the fiber grating proposed by Randy Giles, "Fiber-Grating Sensor for Wavelength Tracking in Single-Fiber WDM Access PON's", Electronics Technology Letters, Vol. 9, No. 4, April 1997, pp 523. Yet another utilizes a narrow-band reflective fiber grating at a temperature sensor at the remote receiver.
These proposed devices are either complicated mechanically, and require a different monitoring element for each laser, or they rely on an external reference to maintain the laser wavelength.
Accordingly, there is a long felt need for a simple device for simultaneously monitoring and controlling laser wavelength for a plurality of lasers which is relatively simple, highly accurate, and which is substantially unaffected by temperature.
There is shown, in our co-pending Israel patent application filed together herewith, a device for monitoring and controlling the wavelength of a single fixed wavelength cooling laser, the device including a wavelength division demultiplexer (WDM) filter arranged to receive a portion of the output laser power of the laser and divide the laser output between the two filter outputs, a photoreceiver arranged to measure the power at each filter outlet, an processor arranged to compute the ratio of the power at the two filter outputs, apparatus to compare the computed ratio with a predetermined reference ratio, and apparatus to adjust the wavelength of the laser in response to the comparison.